Method for cryogenically separating a natural gas stream

ABSTRACT

A method for cryogenically separating a natural gas supply stream into a gas containing the most volatile compounds of the supply stream, and a liquid product containing the heaviest compounds at least including the following. Introducing an at least partially condensed stream into an absorption column at an introduction stage in the lower part of said absorption column, thus producing, at the top, a gaseous stream that contains the most volatile compounds and, the bottom, a liquid product. Introducing the liquid product into a fractionation column in order to obtain, in the bottom of the fractionation column, a liquid product that contains the heaviest compounds of the supply stream and, at the top of the fractionation column, a distillate that is at least partially condensed in a second heat exchanger system

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application No. PCT/FR2017/053045, filed Nov. 8, 2017, which claims priority to French Patent Application No. 1660774, filed Nov. 8, 2016, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds of the feed stream and into a liquid product containing the heaviest compounds of the feed stream.

During the exploitation of natural gas deposits, numerous stages may be provided. A relatively conventional stage after the drying and the withdrawal of the impurities is the separation of the liquids associated with the natural gas (NGLs).

It is often desirable to separate the heavy hydrocarbons, or more generally the NGL (Natural Gas Liquids), from the natural gas, for example such as ethane, butane, propane or C5+ and C6+(that is to say, having at least five carbon atoms and having more than six carbon atoms) hydrocarbons.

This stage can have many advantages but often it is a matter of upgrading various products (ethane, propane, and the like) which are generally sold at a much higher price than the natural gas product. It is in particular common to sell hydrocarbons having a least three carbon atoms as propane, butane and condensate products.

Many industrial installations have been described which make it possible to fractionate gas feedstocks into a residual gas containing the most volatile compounds of the feedstock and into a liquid product containing the heaviest compounds of the feedstock, this being done for the purpose of obtaining, in said liquid product, a given component of the feedstock with a high degree of recovery.

In this regard, mention may be made, for example, of the recovery of liquefied petroleum gas (hydrocarbons therein having three or four carbon atoms) from natural or refinery gas, the recovery of ethane intended in particular to feed steam cracking units, or the desulfurization and the gasoline extraction of natural gases by recovery of the sulfur-comprising compounds, such as carbon oxysulfide and mercaptains Several technologies exist for producing hydrocarbons having at least three carbon atoms from natural gas.

One of the most effective is a process employing a two-column turbo-expander in which the first column is an absorber dedicated to forcing the recovery of as much propane as possible and the second column is a de-ethanizer.

The condensation of the de-ethanizer top stream is often carried out in part with the fluid coming from the absorber bottom. This fluid exits partially evaporated in order to enter the main exchange line requiring a two-phase introduction.

This renders the process very complex in order to provide good distribution in this heat exchanger.

Such a process is described in the documents U.S. Pat. Nos. 4,690,702 and 5,114,450.

The inventors of the present invention have thus developed a solution which makes it possible to solve the problems raised above.

SUMMARY

A subject matter of the present invention is a process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds of the feed stream and into a liquid product containing the heaviest compounds of the feed stream, comprising at least the following stages:

-   -   Stage a): at least partial condensation of a natural gas feed         stream in a first heat-exchange system;     -   Stage b): introduction of the at least partially condensed         stream resulting from stage a) into an absorption column at an         introduction level located in the lower part of said absorption         column, said absorption column producing, at the top, a gas         stream containing the most volatile compounds and, at the         bottom, a liquid product;     -   Stage c): introduction of the liquid product resulting from         stage b) into a fractionation column in order to obtain, in the         fractionation column bottom, a liquid product containing the         heaviest compounds of the feed stream and, at the fractionation         column top, a distillate, at least partially condensed in a         second heat-exchange system;     -   Stage d): introduction, at a level located in the upper part of         the absorption column, of the gas phase of the condensed         distillate resulting from stage c) as feed stream of the         absorption column;

characterized in that the gas stream produced at the absorption column top resulting from stage b) is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.

According to other embodiments, another subject-matter of the invention is: to A process as defined above, characterized in that it comprises a stage, prior to stage d), of condensation of the distillate resulting from the top of the fractionation column in a third heat-exchange system.

A process as defined above, characterized in that all of the gas stream produced at the absorption column top resulting from stage b) is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.

A process as defined above, characterized in that the gas stream produced at the absorption column top resulting from stage b) is separated into several streams, at least one of which is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.

A process as defined above, characterized in that the liquid phase of the condensed distillate resulting from stage c) is used as reflux at the top of the fractionation column.

Thus, the solutions of the process which is a subject matter of the present invention make it possible to dispense with the two-phase entry, or at least limit to a very high LN (liquid/vapor) ratio, of the stream withdrawn at the absorption column bottom before introducing it into a main heat-exchange system prior to its introduction into the fractionation column.

Solution A—The single use of the top stream of the absorption column for condensing the top stream of the fractionation column in a dedicated exchanger has in particular the advantages: the suppression of the two-phase entry of the stream withdrawn at the absorption column bottom into the main exchange line, and the limitation of the delivery pressure of the pump at the outlet of the bottom of the absorption column.

Solution B—Separation of the top fluid of the absorption column into several streams, at least one of which provides the condensation in the top condenser of the fractionation column: this results in a better regulation of the fractionation column top condenser.

Solution C—Condensation of the reflux fluid of the absorption column in a dedicated exchanger by virtue of the top of the absorption column only: this makes possible a simplification of the main exchange line.

The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from natural gas fields, oil reservoirs or a domestic gas network in which the gas is distributed via pipelines.

Generally, the natural gas stream is essentially composed of methane. Preferably, the feed stream comprises at least 80 mol % of methane. Depending on the source, the natural gas contains quantities of hydrocarbons heavier than methane, such as, for example, ethane, propane, butane and pentane and also certain aromatic hydrocarbons. The natural gas stream also contains nonhydrocarbon products, such as H₂O, N₂, CO₂, H₂S and other sulfur-comprising compounds, mercury and others.

The feed stream containing the natural gas is thus pretreated before being introduced into the heat exchanger making possible the first stage of cooling of the process which is a subject matter of the present invention. This pretreatment comprises the reduction and/or the removal of the undesirable components, such as CO₂ and H₂S, or other stages, such as the precooling and/or the pressurization. Given that these measures are well known to a person skilled in the art, they are not described in further detail here.

The expression “natural gas” as used in the present patent application relates to any composition containing hydrocarbons, including at least methane. This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.

The heat exchanger can be any heat exchanger, any unit or other arrangement suitable for making possible the passage of a certain number of streams, and thus making possible at least one system for direct or indirect exchange of heat between one or more liquid coolant lines and one or more feed streams.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 illustrates the diagram of a process according to the state of the art as described in the preamble of the present description.

FIG. 2 illustrates a diagram of an embodiment of an implementation of a process according to the invention.

FIG. 3 illustrates a diagram of a specific embodiment of an implementation of a process according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a feed stream of natural gas 1 is introduced into a main heat exchanger 2 in order to be cooled. The gas thus cooled 3 is partially condensed and introduced into a phase separator 4. The gas phase 5 at the phase separator 4 top is introduced into a turbine 6 in order to recover the expansion energy and to condense a portion of the stream 5, and is then introduced into an absorption column 7 comprising a lower part 7′ and an upper part 7″. The liquid phase 8 at this phase separator 4 bottom is introduced after expansion 9 into the absorption column 7. The absorption column produces a liquid 10 at the column bottom and a residual gas 11 at the column top. The liquid 10 is reheated in a heat exchanger 12 in which it is partially evaporated. The stream thus reheated 13 is subsequently introduced into the main exchanger 2; this introduction 13 is then strongly a two-phase introduction.

At the absorption column 7 top, the residual gas 11, which contains only the products more volatile than ethane, is reheated in the main heat exchanger 2; the stream which results therefrom 14 is subsequently compressed and sent to a treatment unit A.

The stream 13′ at the heat exchanger 2 outlet resulting from the bottom of the absorption column 7 is introduced into a fractionation column 15. This column 15 produces, at the bottom 16, a reboiled liquid product 18 using a reboiler 17 in order to obtain a liquid rich in propane and depleted in ethane. A gas 20 is produced at the fractionation column 15 top 19. This gas 20 is condensed in the heat exchanger 12 and the product 21 which exits from this exchanger 12 is introduced into a phase separator 22. The gas phase 23 at the top of the phase separator 22 acts as reflux in the absorption column 7. The liquid 25 at the bottom of the phase separator 22 acts as reflux 26 at the top of the fractionation column 15. A pump 30 is necessary to pump the liquid 25.

The problem related to the two-phase introduction of the stream 13 into the main heat exchanger 2 is solved by the process which is a subject matter of the present invention.

This is because, in FIG. 2, the residual gas 11 at the absorption column 7 top which contains only the products more volatile than ethane is reheated in a heat exchanger 27 located immediately downstream of the top of said column 7. The gas thus reheated 28 at the outlet of the heat exchanger 27 is then introduced into the heat exchanger 12 at the top of the fractionation column before being introduced into the main exchanger 2 in order to constitute the stream 14 subsequently compressed and sent to a treatment unit A.

Unlike what is illustrated in FIG. 1, the liquid stream 10 at the bottom of the absorption column 7 is pumped using a pump 29 and then directly introduced 13 into the main exchanger 2 in order to form the stream 13′ which is sent to the fractionation column 15.

The advantages of such a process are as follows: Energy efficiency: the pressure of the absorption column 7 is thus maximized.

Simplicity of the exchangers: none of the three heat exchangers 2, 27, 12 has two-phase introduction; the temperature differences in cold fluids and hot fluids are reasonable (i.e., less than 25-30° C., differences beyond which exchangers of brazed aluminum type might be damaged).

Alternatively, other configurations are possible, such as the following, for example: the heat exchanger 12 can be fitted inside the fractionation column 15. The exchanger 27 can for its part be fitted directly above the absorption column 7. The advantage with respect to installing it on the ground is that of avoiding a pump for lifting the reflux.

Another embodiment is represented diagrammatically in FIG. 3. In comparison with the diagram of FIG. 2, the modification consists of a separation of the fluid 11 at the top of the absorption column 7 into several streams 11′ and 11″. The stream 11′ provides the condensation in the condenser 12 at the to fractionation column 15 top.

The stream 11′ is introduced into the condenser 12 at the fractionation column 15 top and is then introduced into the main exchanger 2. The stream 11″ is directly introduced into the main exchanger 2.

The bottom liquid 10 of the absorption column 7 is pumped and then directly introduced into the main exchanger 2.

A control valve can precisely control the fraction sent to the top of the fractionation column 15, making possible precise and effective control of the unit.

Advantage

Better regulation of the fractionation column 15 top condenser 12.

Minimization of the number of items of equipment while maintaining a single-phase introduction of the liquid 13 from the absorption column into the heat exchanger 2.

Alternatively, other configurations are possible, such as the following, for example: the heat exchanger 12 can be fitted directly above the absorption column 7. The advantage with respect to installing it on the ground is that of avoiding a pump for lifting the reflux.

In addition to this, the invention can advantageously be combined with an integration between the columns and the exchangers. In the case corresponding to FIGS. 2 and 3, a configuration which makes it possible to integrate, in one and the same module, the column 15 and the exchanger 12, while avoiding the use of the pump 30, for example, and while avoiding a shell dedicated to the exchanger 12 is provided. In that case, said module is characterized in that a separator is installed directly above the fractionation column 15, above which separator a condenser is installed. The condenser is connected to the top of the fractionation column and to the separator on the condensation side. The bottom of the separator is connected to the fractionation column (indirectly with a valve between the two, typically). Module is thus understood to mean a single structure comprising the column 15, the separator and the heat exchanger 12.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-5. (canceled)
 6. A process for the cryogenic separation of a natural gas feed stream into a gas containing the most volatile compounds of the feed stream and into a liquid product containing the heaviest compounds of the feed stream, comprising at least the following stages: Stage a): at least partial condensation of a natural gas feed stream in a first heat-exchange system; Stage b): introduction of the at least partially condensed stream resulting from stage a) into an absorption column at an introduction level located in the lower part of said absorption column, said absorption column producing, at the top, a gas stream containing the most volatile compounds and, at the bottom, a liquid product; Stage c): introduction of the liquid product resulting from stage b) into a fractionation column in order to obtain, in the fractionation column bottom, a liquid product containing the heaviest compounds of the feed stream and, at the fractionation column top, a distillate, at least partially condensed in a second heat-exchange system; Stage d): introduction, at a level located in the upper part of the absorption column, of the gas phase of the condensed distillate resulting from stage c) as feed stream of the absorption column; wherein the gas stream produced at the absorption column top resulting from stage b) is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.
 7. The process according to claim 6, further comprising a stage, prior to stage d), of condensation of the distillate resulting from the top of the fractionation column in a third heat-exchange system.
 8. The process as claimed in claim 6, wherein all of the gas stream produced at the absorption column top resulting from stage b) is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.
 9. The process as claimed in claim 6, wherein the gas stream produced at the absorption column top resulting from stage b) is separated into several streams, at least one of which is employed in order to condense, in the second heat-exchange system, the distillate resulting from the top of the fractionation column.
 10. The process as claimed in claim 6, wherein the liquid phase of the condensed distillate resulting from stage c) is used as reflux at the top of the fractionation column. 